Every year there has been 1 soil question and 2 biome questions or vice versa. There is no rule anywhere saying that 3 soil questions can't come up but it's pretty unlikely. For most people the biome essays are much easier than the soil essays. If you are pushed for time or just want to lighten the workload, I would have 3 biome essay really well prepared and have one soil essay as a fallback. The rainforest biome is probably the least difficult biome to study as a lot of it is information that is already known. If you are studying the tropical rainforest it makes the most sense to study the latosol as your soil type. I'd have an essay ready on the General Characteristics and the Factors that Influence them. Good luck!